Microresonators, vibration sensors, vibrating gyroscopes and microelectro-mechanical systems, for example made of silicon, may be produced at low cost and in large numbers using methods derived from microelectronics. Due to the small dimensions of the components, these vibrators must be operated in a vacuum. In the case of the resonators, the pressure within the cavity typically ranges from about 1*10−4 mbar to about 1 mbar.
Since hermetically sealed housing technology based on individual devices is comparatively expensive and additionally necessitates the devices to be protected already during the sawing of the silicon wafers, encapsulation of the resonators as early as at the wafer level is widespread. In this context, the wafer which includes the device has a second silicon wafer provided with respective depressions placed thereon, which is finally bonded to the first wafer under vacuum in a hermetically sealed manner. Various methods are contemplated as the interconnection technique used: on the one hand, the wafers may be joined using a glass or metallic solder, on the other hand, the wafers may also be sealed by means of anodic bonding.
Prior to the actual bonding, the wafers are adjusted and fixed within a clamp. Thin spacers between the wafers ensure that the wafers do not contact one another directly. Once the entire clamping arrangement has been introduced, the space around and between the wafers is evacuated, the top and bottom wafers are heated up by means of heating plates, subsequently the wafer surfaces are made to contact one another (once the spacers have been removed), and eventually, the actual bonding is performed.
The thickness of the spacers used ranges from 50 to 500 μm. However, the distance between the wafers already ranges within a dimension which approximately corresponds to the mean free path of the gas molecules at pressures of below 1 mbar, so that diffusion processes primarily determine the behavior of the gases within the spacings of the wafers, as a result of which it becomes clearly more difficult and more time-consuming to remove the gases.
Due to the small distance between the wafers and the comparatively unfavorable ratio between the internal volume and the interior surface within the cavities within the semiconductor devices, for example sensors, experience has shown that it is difficult to join the wafers such that an internal pressure of clearly below 1 mbar may be achieved.
To aggravate the situation, the set pressure within different cavities of the semiconductor devices arranged on the wafer is not constant due to the comparatively slow outgassing of the surfaces between the wafers. While the cavities with the semiconductor devices frequently acquire pressures of clearly below 1 mbar at the wafer edge, the vacuums obtained within the various cavities, which are located further inward within the wafers, are mostly clearly worse. Thus, a pressure distribution arises during the evacuation of the wafers prior to the bonding, the minima of said pressure distribution being located at the wafer edge, and their maxima being located in the wafer center. Introducing breakdowns in the top or bottom wafers entails no advantage, since the wafers are in contact with the top and bottom heating plates, respectively, during bonding.
Eventually, it is only by means of an additional getter material within the sensor cavity that low internal pressures within the cavities may be achieved within a process duration and with a uniformity across the wafer which are suitable for mass production. Even though the getter material necessitates additional activation either during or after the actual bonding process, which additional activation extends the time duration of the bonding process, vacuums ranging from 10−4 to 1 mbar may be ensured only by using a getter. Utilization of a getter is also absolutely imperative with regard to the lifetime of at least 10 to 15 years which is usually envisaged.
However, ensuring the vacuum only superficially solves all problems of vacuum encapsulation of the cavities within the semiconductor devices, for example sensors. Actually, semiconductor devices comprising cavities, such as resonantly operated sensors, do not simply necessitate a vacuum below a critical pressure value within the cavity, but, rather, a vacuum within a certain pressure range is necessitated, i.e. gases, as a rule noble gases, which are not absorbed by the getter material, need to be added in order to place the pressure within the cavity within the desired pressure range, or to keep it there.
However, due to the small distance between the wafers and, thus, to the presence of a pressure regime characterized by a diffusion behavior, introducing the gases is as difficult and/or imperfect as have been the previous attempts to achieve the vacuum solely by means of evacuation. In particular, the achievable uniformities of the distribution of the internal pressures within the cavities of the sensors or semiconductor devices across the wafer prove to be insufficient for a necessitated functionality of the devices.
An improvement may be achieved by employing a gas mixture consisting of nitrogen and some fractions of a noble gas, such as argon. Thus, the wafers are exposed to a pressure of about 5 mbar immediately prior to bonding. Utilization of the increased pressure allows a faster pressure compensation also between the wafers, since with this pressure regime, the behavior of the gases is no longer fully determined by diffusion processes. After bonding, the getter absorbs the remaining nitrogen, while the noble gas remains within the cavity, or sensor cavity.
Even though the results achieved with above methods of creating an internal pressure within a cavity yield an improvement in the pressure distribution, or internal pressure distribution, between a number of semiconductor devices by setting a predetermined pressure within the environment prior to bonding the wafers, the results achieved are nevertheless not sufficient for a correct functionality of the devices thus realized.